Squirrel-cage rotor.



MGGOLLUM. SQUIRREL GAGE ROTOR.

APPLICATION FILED 1330.6, 1911.

1,044,217. Patented Nov. 12, 1912.

UNITED STATES PAEENT OFFICE. A

. nun'rou McCOLLUM, or WASHINGTON, msrmcr or coLuMBrA.

. SQUIRREL-CAGE ao'ron.

To all 'wlwmitmay concern.

Be it known that I, BURTON M COLLUM, a citizen of the .United States,and a resident of Washington, District of Columbia, have invented a newand useful Improvement in Squirrel-Cage Rotors, of which the followingis a specification.

My invention relates to the type of alter nating current induction motorhaving a rotor known as the squirrel cage rotor, and has for its objectthe improvement of the operating characteristics of such motors. Inpractice such motors possess certain disadvantages .in that they have anearly constant resistance in the secondary circuit, for which reason,as is well known, it'is impossible to secure in one'motor both highstarting torque and highrunning efficiency, together with close speedregulation. Further, such motors as ordinarily constructed have aninherently low power factor, which also is objectionable in practice.With a view of overcoming the first of these diiiiculties, it hasheretofore been proposed to use as short circuiting rings of the rotorthin sheets of metal having a positive temperature coeificient ofresistance, and adapt ed to be heated to a high temperature by thestarting currents'of the motor. While it is true that this device has atendency to produce a higher resistance at starting than un- 'derrunning conditions, it has not been pos sible with the constructionheretofore used to obtain anything like the variation of resistancedesired. This is due chiefly to the fact that the short circuiting ringswhich form a part of the rotor circuit are of necessity of considerablelength, and' unless they are, made of large cross section they willintroduce a high resistance into the rotor circuit even under runningconditions, and this causes the motor to operate at a low efiiciency andwith poor, speed regulation. If, on the other hand, the short circuitingrings are made of sufiiciently large cross section to give the desiredlow resistance they will contain so much metal and their capacity forabsorbing heat will be so great that they will heat up very slowly whenthe current is turned on, thus causing the motor to start very slowlyunder heavy loads or to fail to start'altogether. Further, for thereason mentioned above, it is impossible with this construction to useanything but a metal having a high conductivity such as copper,'forthese end rings, so that the Specification of letters ratent.Application filed December 6, 1911. Serial No. 664,219.

amount of metal may be kept small, but since copper and all otherknownmaterials having'a high conductivity, have also a rel atively' lowtemperature coefiicient of resistance, it will readily be seen that there sults obtained by this construction cannot be" adequate. In aprevious application, Serial'No. 612,118, filed Mar. 3, 1911, I havedescribed one means of avoiding this'difiiculty, which means. consistsessentially in inserting permanently in the short circuiting devices ofthe'rotor circuits, resistance elements of the type described below. Ihave now discovered other and simpler means by which the abovementioneddifiiculties may be avoided, and I am thereby enabled not onlyto produce a motor -havin at once a high starting torque and high eciency, but owing to the peculiar properties of the re- Patented Nov.12, 1912'.

sistance devices used by. me the power factor v of the motor undernormal running conditions is greatly improved, and the maximum torquewhich the motor is capable of exerting is, materially increased.Further, owing to the greater mechanical simplicity of the resultingstructure, the cost of the rotor is materially reduced. A full andcomplete description of my invention is contained in the annexedspecification, reference being made to the accompanying drawings.

My invention consists essentially in inserting in each of the conductorbars of the squirrel cage winding a resistance element having arelatively small cross section anda positive temperature coefficient ofresistance, preferably one characterized by an abrupt rise of resistanceat a critical value of current due to the heating effect of the currentin said resistance element.

Of the drawings: Figure 1 isa side elevation partly in section of therotor of the motor towhich my invention applies, showing an arrangementof the resistors in the squirrel. cage winding that I have foundelement. Fig. 5 shows another form of re sistance element and a methodof connecting the same to the conductors of the squirrel cage.

In thus inserting the resistance elements in' series with the conductorbars of the- 100 satisfactory. Fig. 2 is an end elevation' squirrelcage, two important advantages are obtained. It is notthen necessary touse a fixed length of resistance element as is the case when theresistance element constitutes the short circuiting ring of the rotorwinding, but it is permissible to use a resistance element as short asdesired. For this reason I am able to make the resistance elements ofvery small cross section and, at the same time, to make them ofsuchshort length that the resistance under norm'al running conditions willnot be high@ enough to affect seriously the efficiency and speedregulation of the motor. In this way j Iain enabled to use metals havinga much 1' higher specific resistance than has hereto- E fore beenpracticable, and can therefore use 5 those metals which give thegreatest increase of resistance with rise of tempera- 1 ture, and at thesame time the quantity of 1 metal in the resistors can be kept so small1 that it heats very rapidly when the current l is turned 011, thusproducing a very rapid E rise in resistance and a correspondingly rapidincrease in the torque. Further, by I thus inserting the resistanceelements in the conductor bars of the rotor, the current carried by eachelement will be relatively small compared to the current in the shortcircuiting rings, and the resistance elements can therefore more readilybe made of small cross section and a more rapid rise in resistance issecured. Also, inthis case a single short circuiting ring only isrequired on each end of the rotor and a better mechanical constructionis obtained.

I have experimented with many sub-E stances as resistance elements andhave 1 found that the magnetic metals, iron, nickel and cobalt are muchbetter than any others, in that they show a very great and abrupt, riseof resistance at a certain well defined critical value of current. It ischaracteristic of these metals that when used in comparatively purestate they show a relatively low and only slightly changing resistanceas the current is increased up to a certain critical value, at whichpoint any further increase in current causes the resistance to rise veryquickly to a manifold value. My invention consists further in soproportioning the resistance element that the normal load currents inthe rotor will be somewhat below the critical value necimportantimprovements in the operating characteristics are obtained. Under normaloperating conditions the frequency of the alternating currents in thesecondary circuit of an induction motor is very low. Since thetemperature, and therefore the resistance of the resistance elements,tends to vary with the current strength, it is evi-- they can be made ofvery small dimensions as described, their capacity for absorbing heat isso small that relatively great pulsations in resistance will occur withthe low frequencies that usually prevailin the secondaries of inductionmotors in practice. Herefrom follows the important advantages referredto above.

Since the heat. in the resistance element is increasing and theresistance therefore rising as long as the square of the current in theelement is above its meah value, ap-

proximately speaking, it follows that the resistance of the element willcontinue 'to increase for some time after the current has passed throughits maximum value, and. as a result of this, the maximum of current ismade to occur sooner than it would if the resistance were unvarying. Theeffective current in the rotor is therefore advanced in phase, acondition that improves the power factor of the motor and enables it toexert a higher maximum torque as mentioned above. Thls pulsation inresistance is greatly augmented by using materials having a relativelygreat tendency to increase their resistance with increasing current,such as the magnetic materials, and these metals are therefore bestsuited to securing all of the advantages mentioned above, viz: highstarting torque combined with high efiiciency, good speed regulation,improved power factor and high maximum torque.

If iron or cobalt be not worked at too high a temperature they are quitestable in air, and'will, under such circumstances,.have an indefinitelife. Then, however, it is desired to operate them at very hightemperatures during the starting period in order to obtain the maximumincrease in resistance, they may gradually oxidize and deteriorate.Nickel is much mor resistant to oxidation than iron or cobalt, and atmoderately high temperatures either nickel alone, or iron or cobaltcoated with'nickel makes a more satisfactory resistance element. When itis desired to push the temperatures at starting to still higher'values,

whichdoes not oxidize readily at the, temperature desired. This canreadily be accomplished by numerous well known methods, Coatings of-goldor platinum are pure ticable examples ofthis embodiment of my invention.

Referring particularly to Figs. 1 and 2, 1 designates the laminated coreof the rotor, 2 the conductor bars of the squirrel cage winding embeddedin the slots of the rotor core, and 3the short circuiting end rings ofthe squirrel cage. It will be observed that in the case illustratedthere are inserted on each end of the rotor in alternate conductor bars,resistance elements 4, of very-short lengthy and greatly reduced crosssection, these being proportioned in accordance with the principles setforth .above. It should be understood that this alternate placing of theresistance elements on either end of the rotor is not essential to thesatisfactory operation of the motor from an electrical standpoint. In so"far as the electrical operation is concerned, the resistance elementsmight all be, placed on one end of the-rotor, or they might be groupedin any 'ofa variety of ways that readilysuggest themselves. ,It isdesirable, however,-.that

the heavy short circuiting rings 3, be each connected directly to someof the conductor bars in order to secure ample'mechanical strength andrigidity. If the rings, 3, be

otherwise suitably braced and held in position, any othergrouping of theresistance elements in the conductor bars will be satisfactory.

, In the construction illustrated in Figs. 1 and 2, it will be'seen thatthe resistance ele ments are not integral with the conductor bars, butconsist of separate pieces of metal fastened on one. end to theconductor bars, and on the other, to the end rings by soldering or othersuitable means. In this case the resistance element may be of the sameor different material from that of the conductor bar.

In Fig. 4 is shown a modified construction of the resistance element inwhich a-shortportion, 4, near the end of the conductor bar 2 is greatlyreduced in cross section,

iii

the bar being connected to the end ring3,

by means of the screw In Fig. 5 is shown a still further modification ofthe resistance element. Here, the resistance element t is not integralwith the conductor bar 2", and may be of the same or different material,and is connected therewith at one end by means of the screw 6. The otherend of the resistance element 4: is connected to the lug 7 by means ofthe screw 8, and this lug is in turn connected rel cage winding},

to the-,sliort circuitingj ring 3 by means of the screws 9. It will beunderstood that any combination of the essential elements of theconstruction shown may be used with equal .satisfaction, the resistanceelements being connected at either or both ends by means of solder,screws, rivets, bolts or clamps, or by any other suitable means designedto give a good and permanent electrical and mechanical connection.

In the annexed claims, I cover broadly the use of resistance elements ofreduced cross section and positive temperature coefficient of resistanceinserted in the conductor bars ofthe squirrel cage, and claimspecifically the use of iron as the essential ele ment insuch'resistors. That portion of the above disclosure which relates tothe use of cobalt and nickel as the essential elements in theresistorsis made the basis of two sepa rate applications, .Serial Nos. 684,419and 684,420, filed Mar. 18, I912, both entitled Imp'rovementin inductionmotors.

Iclaim:

1 .In an induction motor having a squira series resistance in each ofthe bars of t e winding, said resistances comprising portions of reducedcross secnon having a positive temperature coefiicient of res stance,and characterized by an abrupt rise of resistance at a critical value ofcurrent. v 2. In an induction motor having a .squirrel cage winding, aseries resistance in each of the bars of the winding, .said resistancescomprising port ons of reduced cross secti'on having a positivetemperature coeflicient of resistance, and characterized by "anabruptrise of resistance at a critical current value, and soproportioned that the normal starting currents in the rotor bars will beabove the critical current value, and the normal load currents rentvalue. I

In' an induction motor having a squirrel cage winding, a seriesresistance in each of the bars of the winding, said resistancescomprising portions of reduced cross section composed. of a magnetic.material, and

adapted to be. heated to a relatively high temperature by the startingcurrents in: the

rotor bars.

4. In an induction motor having asquirrel cage winding, a seriesresistance in each of the barsof the winding, said resistancescomprising portions of reduced cross. section comp'osed of iron, andadapted to be heated to a relatively high temperature by the startingcurrents in the'rotor bars.

5. In an. induction motor having a squirrel cagewinding, a seriesresistance in each of the bars of the winding, said resistancescomprising portions of reduced cross section.

having a positive temperature coeflicient of resistance and coated witha material subbelow the critical curof the bars of the Winding, saidresistances comprising portions of reduced cross section, composed oi:magnetlc material coated with a material substantially non-oxidizable atthe temperatures normally reached b y the resistances.

7. In an mductlon motor, and 1n combination, a squirrel cage Winding, aseries resistance in each of the bars of said Winding, said resistancescomprising portions of reduced cross section, composed of a magneticmaterial coated with nickel,

8. In an induction motor, and in combination, a squirrel cage Winding, aseries resistance in each of the bars of said winding, said resistancescomprising portions of reduced cross section, composed of'a conductor ofiron coated with nickel.

' BURTON MGCOLLUM.

Witnesses v JESSIE C. MCCOLLUM, R. H. KEssNER. a

